Fully closed-cycle underwater breathing apparatus (CCUBA) or alternatively known as “closed-circuit rebreathers”, or “CCR” offers distinct advantages over the more common open-circuit (SCUBA) systems, such as reduced bubble noise, extremely high gas usage efficiency, and optimized breathing gas composition.
These advantages derive from the fact that the exhaled breathing gas is recycled, filtered of carbon dioxide, replenished with oxygen, and returned to the diver for breathing again. The lack of bubble noise and the increased gas efficiency of a CCR both result from the fundamental function of recycling the breathing gas. The optimized breathing gas composition results from the fact that the oxygen control system of a CCR maintains a constant partial-pressure of oxygen (rather than a constant fraction of oxygen, as in conventional open-circuit SCUBA).
The partial pressure of a gas is a function of the fraction of the gas multiplied by the ambient pressure. As a diver descends and the depth increases, the ambient pressure also increases. Thus, for a given fraction of oxygen, the partial pressure increases as the depth increases. If the oxygen partial pressure exceeds a certain threshold (approximately 1.4 bar) the risk of hyperoxia-induced seizure and other “oxygen toxicity” symptoms is considered unsafe for the diver. For example, the maximum safe depth at which a diver can breathe a mixture containing 50% oxygen is about 18 meters. On the other hand, the lower the oxygen concentration, the greater the concentration of non-oxygen gas constituents, such as nitrogen or helium. It is these non-oxygen components of the breathing mixture that lead to problems of decompression sickness (DCS), also known as “the bends”, which include symptoms ranging from pain in the joints, to paralysis, to death. To maximize the amount of time that can be safely spent at any given depth, the non-oxygen portions of the breathing gas should be kept to a minimum; which means that the oxygen should be kept to its maximum safe limit at all points during the dive.
Thus, the advantage of CCR over conventional open-circuit SCUBA in terms of optimized breathing gas composition results from the fact that a CCR can maintain the maximum safe partial pressure of oxygen (PO2) throughout all depths of a dive, thereby minimizing the concentration of non-oxygen gas constituents—leading to increased allowed time at any give depth and/or reduced risk of DCS.
But this advantage comes at a cost. Whereas the breathing mixture for a conventional open-circuit SCUBA diver is fixed based by the composition of the gas in the supply cylinder, the breathing mixture in a CCR is dynamic. Although it is this dynamic mixture capability that affords CCR one of its primary advantages, a failure of the oxygen control system can be extremely dangerous. A malfunction that allows the PO2 to get too high places the diver at risk of a hyperoxia-induced seizure, almost certainly causing the diver to drown. A malfunction that allows the PO2 to get too low will lead to hypoxic-induced blackout, causing the diver to drown and/or suffer severe brain damage. Therefore, perhaps the most critical aspect of any CCR design involves the reliability of the oxygen control system.
Most modern CCRs incorporate one or more electronic oxygen sensors that directly measure the PO2 of the breathing gas and as well, have an onboard computer processor to analyze the data and to advise the user of the status of the system by means of some sort of display, either digital or analog—typically mounted on the user's wrist and connected to the computer via an electrical cable. In the event of a failure of such electronic sensing and advisory systems it is current practice in CCR diving to have some sort of external open-circuit (traditional) Scuba system available with which to abort to the surface. Finding this auxiliary breathing mouthpiece in the event of an emergency or panic can be fatal if the user is not able to immediately and exactly locate the spare mouthpiece, which is a physically separate object usually clipped either to the emergency gas source or somewhere on the user's life support harness. Experience, and actuarial statistics, support the claim that locating and activating this external mouthpiece is not guaranteed.
One solution to this problem is described in U.S. Pat. No. 5,127,398 Stone and U.S. Pat. No. 5,368,018. The solution was then to design a combined mouthpiece that contains the functionality of both open-circuit and closed-circuit breathing systems such that in the event of an emergency with the closed-circuit system the user can make a simple change to the state of the mouthpiece system to convert directly from closed-circuit to open-circuit operation in the event of an emergency and without ever having to remove the mouthbit.
An additional function that is required of all CCR breathing apparati is the ability to add a breathable gas (i.e. a “diluent” gas) to the compliant volume of the CCR when that compliant volume drops below an amount needed to fill the user's lungs upon inhalation. There are many situations where such an action will be required and it is customary to provide an independent system that consists of a special low pressure regulator that is attached advantageously at a location on the CCR compliant volume (known as a “counterlung”) and provides access to a supply of breathable gas, usually from a high pressure tank equipped with a high pressure regulator that thence provides a flow of gas to the low pressure regulator, which is typically in the 8 to 12 bar pressure range. The special low pressure volume compensation regulator is known as an “ADV” (automatic diluent-addition valve).
In the patent publication of GB 2,340,760 A, a mouth piece for a CCR is disclosed which comprises a switch to switch between open circuit breathing and closed circuit breathing. The mouth piece further comprises valve means, which is said to be able to operate automatically and to permit the introduction of breathable gas from a separate source into the system. The mouth piece also comprises manually operable valve means for the addition of a diluent gas. Over all, the mouth piece can be said to provide an automatic diluent function, a manual diluent function and a valve emergency open circuit breathing valve, combined in a single unit.
However, the mouth piece just described provides for several drawbacks, for instance, the sensitivity of the trigger mechanism for the automatic diluent function is changed by altering the value of a spring. Hence this is not a very practical solution, especially not for a diver submerged in water.
There is a need for a more practical mouth piece which provides at least a part of the above mentioned advantages, while at the same time eliminating or minimizing at least a part of the mentioned drawbacks.